Here, “array antenna” is understood to mean an antenna able to operate in transmission and/or reception and comprising an array of sub-arrays of at least one radiating element and control means suitable for controlling by means of active chain(s) the amplitude and/or the phase of the radiofrequency signals to be transmitted (or in the opposite direction, received from space in the form of waves) by each of the sub-arrays so that they transmit (or receive) radiofrequency signals according to a chosen pattern. Consequently, this will equally well involve so-called direct-radiation array antennas (often denoted by their acronym DRA), active or more rarely passive ones, and “reflector-array antennas” (or “reflectarray antennas”).
As known by the person skilled in the art, certain array antennas, such as for example the direct-radiation antennas with amplifiers distributed just behind the radiating elements, make it possible to operate in multibeam mode, this being a basic property required for example within the framework of multimedia missions in the Ka band (18.2 GHz to 20.2 GHz in transmission or 27.5 GHz to 30 GHz in reception), or to reconfigure beams in flight, for example in the Ku band (10.7 GHz to 12.75 GHz in transmission or 13.75 GHz to 15.6 GHz in reception).
However, these arrays exhibit two main drawbacks. They in fact require a large number of active chains once the coverage zone has to be decomposed into very fine beams (or “spots”) and there is a strong constraint of isolation between nearby zones so as to be able periodically to reuse one and the same frequency sub-band. Furthermore, the low energy efficiency (determining criterion in transmission) of the amplifiers included in their active chains in the presence of broadband multi-carriers gets worse when they are not used at their optimal power level. This results in fact from what is called apodization (also known as “taper”) which is indispensable when one wishes to obtain fairly weak sidelobes (of the antenna patterns). It is recalled that apodization is a technique consisting in placing more energy at the center of the array than at its periphery.
A third drawback may be added to the previous two main ones when in the presence of a strong constraint of isolation between nearby zones on account of frequency reuse. Specifically, the “gentle” degradation in performance when a few active chains become faulty (progressively during a mission) often becomes unacceptable when the percentage of faults becomes significant. To remedy this drawback it is admittedly possible to envisage a conventional redundancy of sub-arrays of radiating elements, of the type “2 for 1”, or “3 for 2”, or else “10 for 8”, but this entails unacceptable complexity for large arrays, and a significant increase in mass (particularly penalizing drawback for antennas aboard satellites).
To attempt to remedy the aforesaid drawbacks, there has been proposed in patent document FR 2762937 a sparse array antenna with “cold redundancy”. This solution consists in providing at chosen locations of the array a restricted number of substitute sub-arrays and of associated active control chains, which are used only in the event of a fault with one or more active control chains. The locations of these substitute sub-arrays are chosen so that transmission and/or reception continues to meet the requirements: to a first approximation, the apodized distribution law for the energy must remain overall similar before and after activation of some of the redundancies.
When a substitute sub-array is not used, it forms a transmission and/or reception void in the array, which is taken into account during antenna optimization. However, the presence of a considerable number of voids in the array lowers the directivity of the antenna for a given exterior dimension. Additionally, because of the regular meshing of the array before the definition of the voids, if one wishes to obtain weak sidelobes (to prevent in particular the “array lobes” due to the periodicity from interfering in the useful angular domain) it is compulsory to use sub-arrays with a small number of radiating elements, so that the total number of sub-arrays can be only slightly reduced.
Since no known solution is entirely satisfactory, the aim of the invention is therefore to improve the situation.